Enhancements in OpenMP 2.0

Transcription

1 Enhancements in University of Stuttgart High-Performance Computing-Center Stuttgart (HLRS) Slide 1 Outline Timeline Clarifications/Modifications New Features Slide Enhancements in

2 Timeline OpenMP 1.0 for Fortran released October 1997 OpenMP 1.0 for C/C++ released at October 1998 OpenMP 1.1 for Fortran released at November 1999 for Fortran released at November 2000 for C/C++ in preparation Slide 3 Clarifications interface definitions for OpenMP runtime routines have been added an OpenMP compliant implementation must documents its implementation-defined behavior clarification of implied flush directive Recycling of thread numbers is clarified ( if dynamic threads are disabled, the threads keep the same number on subsequent parallel regions) initialized data must have the SAVE attribute, like in Fortran 95 Slide Enhancements in

3 Clarifications: Implementation-defined behavior See Appendix E of the standard The size of the first chunk in SCHEDULE(GUIDED) default schedule for SCHEDULE(RUNTIME) default schedule default number of threads default for dynamic thread adjustment number of threads used to execute nested parallel regions atomic directives might be replaced by critical sections behavior in case of thread exhaustion use of parameters other than OMP_*_KIND in generic interfaces allocation status of allocatable arrays that are not affected by COPYIN clause are undefined if dynamic thread mechanism is enabled Slide 5 Clarifications: Implied flush directive A FLUSH directive identifies a sequence point at which a consistent view of the shared memory is guaranteed It is implied at the following constructs: BARRIER CRITICAL and END CRITICAL END {DO, SECTIONS} END {SINGLE, WORKSHARE} ORDERED AND END ORDERED PARALLEL and END PARALLEL with their combined variants It is NOT implied at the following constructs: DO MASTER and END MASTER SECTIONS SINGLE WORKSHARE Thread- n m number new value Memory old value Cache CPU Registers? cache flush is needed to load the new value into the CPU Slide Enhancements in

4 New Features Wallclock timers WORKSHARE directive REDUCTION on array names NUM_THREAD clause _OPENMP preprocessor macro Nested lock routines like in C/C++ Reprivatization of variables is allowed Directives may contain comments COPYPRIVATE is a new modifier on END SINGLE THREADPRIVATE may be applied to common blocks COPYIN on common blocks Slide 7 New Feature: Wall clock timers Portable wall clock timers similar to MPI_WTIME DOUBLE PRECISION FUNCTION OMP_GET_WTIME() provides elapsed time START=OMP_GET_WTIME()! Work to be measured END = OMP_GET_WTIME() PRINT *, Work took, END-START, seconds provides per-thread time, i.e. needs not be globally consistent DOUBLE PRECISION FUNCTION OMP_GET_WTICK() returns the number of seconds between two successive clock ticks Slide Enhancements in

5 New Feature: WORKSHARE directive WORKSHARE directive allows parallelization of array expressions and FORALL statements Usage:!$OMP WORKSHARE A=B! Rest of block!$omp END WORKSHARE Semantics: Work inside block is divided into separate units of work. Each unit of work is executed only once. The units of work are assigned to threads in any manner. The compiler must ensure sequential semantics. Similar to PARALLEL DO without explicit loops. Slide 9 New Feature: Reduction on Arrays REDUCTION clause may be applied to arrays Example:!$OMP PARALLEL DO REDUCTION(MAX:M) DO I=1, 100 M=MAX(M,N(I)) END DO Deferred shape and assumed size arrays are not allowed! Slide Enhancements in

6 New Feature: NUM_THREAD clause The NUM_THREAD clause on parallel regions defines the number of threads to be used to execute that region Example:!$OMP PARALLEL NUM_THREADS(scalar integer expression) block!$omp END PARALLEL can also be used on combined parallel work-sharing constructs Slide 11 New Feature: _OPENMP If an OpenMP compliant compiler supports a macro preprocessor it has to define the symbol _OPENMP The symbol is of the value YYYYMM YYYY is the year MM the month of the version of OpenMP the implementation supports Slide Enhancements in

7 Summary contains many clarification that reflects the experience made in the last years some improvements/extensions: interface definitions Wallclock timers WORKSHARE directive REDUCTION on array names Some features have direct expressions in C/C++ and can be expected in for C/C++ Slide Enhancements in